Method for cleaning electronic components

ABSTRACT

A degreasing and cleaning method for cleaning circuit boards and the like has a first, wash sump containing a boiling, environmentally friendly wash material and a second, rinse, sump containing an environmentally friendly rinse material, separated by a weir. An anti-splash and surge control fence is mounted on top of the weir to control splashing and spill-over between the sumps. The wash sump contains a plurality of spray wands and a programmed hoist pans the carrier of the boards past the spray wands in incremental steps. After the wash cycle, the carrier is introduced into the second, rinse, sump and then given a final vapor rinse. The temperature ranges in which the apparatus is operated yield optimum cleaning.

FIELD OF INVENTION

This invention relates to a cleaning system for electronic parts and,more particularly, to a method and apparatus for solvent cleaning ofcircuit card assemblies.

BACKGROUND OF THE INVENTION

In the manufacture of circuit card assemblies (CCAs) such as multi-layerceramic cards, the ceramic or other such substrates are populated with avariety of devices, including leadless chip carriers having typicalboard to device clearances of, for example, 0.006 inches and generally aminimum stand-off of 0.004 inches. Inasmuch as solder and solder fluxmountings and connections are most commonly used in mounting andconnecting the various devices to the substrate, the extremely closespacing between device and board traps flux residues that must beremoved to avoid problems in subsequent manufacturing steps, such aswhen the CCAs are conformally coated. The removal of flux residues hasbeen generally accomplished through the use of vapor degreasingequipment.

Typical of such prior art degreasing equipment is a dual sump assemblyin which the first sump contains a degreasing solvent which is heatedand the second sump also contains the same solvent. The first sumpfunctions as a wash bath and the second sump functions as a rinse bath.A suitable conveyor means introduces a basket, for example, containingone or more CCAs into the heated solvent of the first sump. After awashing period, which may be of any suitable duration, the basket isremoved from the liquid of the first sump and lifted into a vapor areaabove the sump through which it is passed to the second sump, where itis rinsed.

In some cleaning applications, it is highly desirable, if not mandatory,that the contaminant level after cleaning not exceed ten micrograms persquare inch on the circuit board. More particularly, the ioniccontamination of the surface of the board, which generally comprises aceramic substrate populated with various devices and is, for example,six inches square, must be lower than ten micrograms (NaCl equivalent)per square inch; ion chromatography requirements by type must be lessthan one microgram per square inch for each of the ions chloride,bromide, and sulfate; and there should be no visible residue left on thesurface under 7× magnification. Heretofore, excellent cleaning resultswhich met the foregoing criteria had been achieved with 1,1,1trichlorethane in both of the sumps of the previously described vapordegreasing equipment. However, that particular solvent is a Class 1ozone depleting substance (ODS-1) CFC covered under the MontrealProtocol, the use of which is restricted by U.S. EnvironmentalProtection Agency regulations, and international treaties.

As a consequence of the foregoing, it is extremely important that therebe a cleaning system for circuit boards that uses environmentally"friendly" materials while meeting or exceeding the aforementionedcleanliness criteria.

SUMMARY OF THE INVENTION

The present invention is a cleaning apparatus and method which involvesco-solvent cleaning wherein the solvents are environmentally friendlyand which meets the specified criteria for cleanliness.

In a preferred embodiment of the invention, a tank has a transverse weirtherein forming first and second sumps. The first sump contains asolvating liquid agent, a mixture of tetradecanoic acid, 1-methylethylester and dipropylene glycol n-butyl ether, which is non ozone depletingand of very low toxicity. Such material is commercially available from,for example, Petroferm Inc. whose designation of the material isSolvating Agent 70 (SA70®). The second sump contains a liquid rinseagent which comprises a mixture of (nonaflourobutyl) methyl ether and(nonaflouroisobutyl) methyl ether. Such material is commerciallyavailable from the 3M™ Company with the designation HFE-7100®. Both thewash solvent and the rinse solvent are environmentally "friendly" andtheir use is not restricted by EPA regulations. In addition, bothmaterials are of low toxicity and only minimally hazardous in use.

It has been found that best results are attainable by boiling thesolvating cleaning mixture in the first sump. To this end, the firstsump contains, or has mounted in proximity thereto, suitable heatingcoils for bringing the mixture therein to a boil. However, the boilingpoint of the solvating agent in the first sump is approximately 350° F.which is considerably higher than is desirable. On the other hand, theboiling point of the rinse material is approximately 140° F., and it canbe used to reduce the boiling point of the mixture in the first sump bythe introduction thereof in measured amounts into the first sump mixturewhich occurs upon heating when the rinse solvent overflows the weir intothe wash solvent. In addition, if there is too great a reduction inboiling point of the wash mixture, measured amounts of rinse mixture maybe taken out of the rinse bath to reduce or stop the overflow. It hasbeen found that a boiling point range for the solvating agent in thefirst sump of approximately 180° F. to 200° F. yields satisfactoryresults, with the range 183° F. to 187° F. yielding excellent results.The temperature range is achieved through the introduction of properamounts of rinse agent on an empirical basis. Thus, temperatures areconsiderably higher than is usual in degreasing or cleaning processes.As a further temperature control, the first sump has a thermocoupletherein for monitoring the temperature of the wash bath to determine ifit is at or near the desired operating temperature, and the signal fromwhich may be directed to a visual indicator and alarm and to a heatingcoil control if desired. Thus, the boiling point of the wash solvent inthe first sump is governed or controlled by the addition or removal ofrinse agent, and the temperature is maintained by the heating coils andthermocouple arrangement.

A hoist or other type conveyor is mounted over the tank for conveying abasket containing circuit boards along the length of the tank. The hoistis programmed to lower the basket into the wash solvent in the firstsump, then to raise the basket and pass it through a vapor volume whichforms above the tank to the rinse sump where it lowers the basket intothe rinse liquid in the second sump. In order that the washing action inthe first sump be enhanced, sub-surface arrays of relatively highpressure, e.g., 50 psig, nozzles are located in the first sump forincreasing the agitation of the wash solvent. The hoist is programmed tomove the basket vertically in incremental steps past the nozzles,thereby panning the basket past the several nozzles and assuring totalcoverage by the nozzles with no residual voids.

When the basket is lowered into the rinse solvent, it displaces arelatively large volume of rinse fluid which tends to spill over theweir into the first sump and cause a relatively violent boilingreaction. In addition, this spill over functions as a purge to removetraces of cleaning solvent introduced by the rinsing process. A fence ismounted along the top of the weir and has spaced holes therein to allowspill over but to prevent too sudden surges of overflow. The fence alsoprevents wash solvent from splashing over the weir into the rinse bath.Also, when a large surge of rinse agent overflows into the boiling washagent, it causes a sudden dramatic increase in the boiling action of thewash agent which tends to flow over the weir and contaminate the rinsematerial, and it also causes sharp decrease in the temperature of thewash agent. As a consequence, the rinse bath remains substantiallyunadulterated and sudden changes in temperature of the wash bath areminimized. After the basket and the boards therein have been rinsed inthe rinse bath, the hoist raises the basket above the tank and the rinsevapor volume to a region where it is cooled, and then the basket islowered into the vapor volume where, because it has been cooled, thevapor, which is virtually pure rinse solvent, condenses on the basketand the boards to perform a final rinse, after which the boards arepassed to an inspection stage.

The system as described is a closed, recirculating system in which pumpsare used to recirculate both the washing solvent and, separately, therinsing solvent, with appropriate filtering where necessary. Preferablythe two sump tank is contained in a protective housing, and, for optimumcleanliness of the product, the entire system is contained, and theprocess performed, within a clean room.

The numerous features of the present invention and the advantages thusrealized will be more apparent from the following detailed description,read in conjunction with the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of the cleaning apparatus of the presentinvention;

FIG. 2 is a perspective diagrammatic view of that portion of theapparatus of the invention embodying the dual sumps and the componentsused therewith;

FIG. 3 is a plan view of the wash sump of the apparatus of theinvention;

FIG. 4 is a front elevation view of the splash fence of the presentinvention;

FIG. 5 is a side elevation view of the fence of FIG. 4 as mounted on theweir separating the two sumps; and

FIG. 6 is a side elevation view of the interior of the tank containingthe two sumps.

DETAILED DESCRIPTION

FIG. 1 depicts the apparatus of the preferred embodiment of theinvention which comprises a cleaning system 11. Inasmuch as cleanlinessis the major consideration, the system 11 is preferably located in aclean room, not shown, although this location is not limiting.

System 11 comprises cabinet or other type container 12 of suitablematerial, such as steel or stainless steel within which is contained atank 13, shown in dashed lines. Tank 13 is shown mounted on supportmembers 14, however, other means for locating the tank within container12 may be used. As is shown in dashed lines, tank 13 is divided into twoadjacent sumps 16 and 17 separated by a weir 18. The details and variouscomponents of the tank 13 and sumps 16 and 17 will be discussed morefully hereinafter, in particular with regard to FIG. 2. Support member19 and 21 support a hoist track 22 above the tank 13 as shown. Mountedon track 22 and adapted to travel therealong is a hoist 23 whichcomprises a control and actuating unit 24, depending cables 26, and ahook 27. In use, a basket 28, preferably of a metallic mesh is attachedto hook 27 by a handle 29. As will be apparent hereinafter, basket 28 isadapted to carry a number, e.g., ten, of circuit boards to be cleaned.The cable and hook arrangement depicted in FIG. 1 is a preferredarrangement, however, it is to be understood that other types ofcarriers for the basket 28 may be used. It is desirable, in any case,that the hoist arrangement allow quick attachment and detachment of thebasket 28, with the attachment securely holding the basket 28. As willbe discussed more fully hereinafter, actuating unit is programmed toraise and lower the basket 28, thereby lowering it into the sumps 16 and17, lifting it, and moving it longitudinally from one sump to the other.In the embodiment of the present invention depicted in FIG. 1, sump 16contains a washing bath and sump 17 contains a rinsing bath, and thebasket is, during the cleaning operation, moved from the sump 16 to thesump 17 by the actuator 24 traveling on track 22. The hoist arrangement23 of FIG. 1, wherein the actuator 24 travels along a track 22 is onlyone of a number of possible hoist arrangements for fulfilling theforegoing requirements and others to be discussed in conjunction withsubsequent figures.

The various components of the cleaning system 11, their location, andtheir relationship to other components are shown more clearly in FIG. 2.Located at the bottom of wash sump 16 are heating coils 31 which areconnected, via line 32, to a temperature monitoring and control unit.The heating coils 31 may be located in a space below the bottom of sump16, as shown in FIG. 1. As discussed hereinbefore, the wash solventmixture (not shown) within sump 16 is boiled, with a desirable boilingtemperature being 183° F. to 187° F. a thermocouple 34 which is immersedin the fluid in wash sump 16 is also connected via wire 36, to unit 33.Unit 33 monitors the temperature of the wash bath in sump 16 and givesan indication through an alphanumeric read out 37 of the operatingtemperature and a color status indicator (green, yellow, red) 38 gives aquick indication of operating temperature. Within sump 16 are mounted,by any suitable means, first and second pairs of spaced spray wands 41and 42, each wand having a plurality of nozzles or openings fordelivering wash fluid under pressure to the space between the pairs 41and 42. A pump 43, which recirculates the wash solvent through conduits44 and 46 supplies the fluid to the wands 41 and 42. The operation ofthe spray wands 41 and 42 will be discussed more fully hereinafter. Thesump 16, during operation, is filled to a point below the top of theweir 18 with the proper wash solvent, i.e., a solvating liquid agentcomprising a mixture of tetradeconoic acid, 1-methylethyl ester anddipropylene glycol n-butyl ether (Petroferm SA70®) which, as pointed outin the preceding discussion, is non-ozone depleting and of very lowtoxicity. In addition, approximately 20-30% of the fluid consists ofrinse agent. This lowers the boiling point to the range of operatingtemperatures. After the apparatus stabilizes thermally, the final targettemperature is attained by manually adding or sub-boiling the rinseagent. The level of the liquid in sump 16 should be sufficiently belowthe top of the weir 18 so that the boiling wash mixture of SA70 andHFE7100 does not tend to spill over into sump 17. On the other hand,sump 17 is filled with the proper rinsing agent, a mixture of(nonaflourabutyl) methyl ether and (nonaflouroisobutyl) methyl ether(3M™ HFE7100) to a point just below the top of the weir. As seen in FIG.2, a pump 47 extracts rinse agent from sump 17 through a conduit 48 andpasses it through a water separator 49 and a filter 51, and returns thecleaned fluid to sump 17 through conduit 52.

A pair of cooling coil 53 is mounted within tank 13 above the sumps 16and 17 and are controlled by suitable means, represented by block 56.The function of the cooling coil will be clear from the followingdiscussion of the operation of the system.

Mounted on top of weir 18 is a fence 57 for controlling overflow offluid between the two sumps 16 and 17, and splash back of fluid betweensumps 16 and 17.

FIG. 3 is a plan view of sump 16 showing the relationship of the boardcontaining basket 28 to the spray wands 41 and 42. Basket 28, which maycontain, for example ten circuit cards, when lowered into the washsolvent by hoist 23, passes between the pairs of wands 41 and 42 which,as pointed out hereinbefore, deliver relatively high pressure spraysfrom their several nozzles 58,58 and 59,59 into the already boilingliquid mixture, which impinge upon the mesh basket and its contents toinsure a proper and thorough washing.

During operation, it is necessary to keep the rinse bath clean, and alsoto prevent too sudden or too great a spill-over of fluid from one sumpto the other. In order to further these desiderata, a fence 57 ismounted on top of the weir 18 by suitable means such as L-shapedbrackets 61. As can be seen in FIG. 5, fence 57 is so located on weir 18that any wash solvent impinging on its face or front surface, drainsdirectly back into the wash bath, thereby minimizing contamination ofthe rinse bath. Fence 57 has an array of holes 62 arrayed across thewidth thereof to control fluid surges. Thus, when basket 28 is loweredinto the rinse bath, thereby displacing an amount of rinse fluid, theholes 62 allow spill-over of the displaced fluid into sump 16, but at acontrolled rate rather than as a large surge. The fence 57 is,therefore, a surge protector or preventor. At the same time, fence 57protects the rinse bath from splashing of the wash bath fluid, which isin a state of agitation, into the rinse bath. Thus, the amount ofcontamination of the rinse bath as a result of such splashing isminimized.

OPERATION

When the sumps 16 and 17 contain the proper levels of bath liquids,e.g., three-quarters full for the wash sump 16 and substantially fullfor the rinse sump 17, the heating elements are turned on and thetemperatures rise until the wash solvent commences to boil. At thispoint, if the boiling temperature is too high, it can be reduced by theaddition of HFE7100®, the material forming the rinse bath, in measuredamounts, until the desired boiling point for the wash bath is reached.If the boiling temperature is too low, HFE 7100® is removed from therinse bath. For a target boiling temperature of 185° F. for the washbath, the temperature of the rinse bath will be approximately 140° F. Atthese temperatures, a vapor zone 63, as shown in FIG. 6 is created overthe sumps, which for the most part is confined between the uppersurfaces of the baths and the cooling coil 53. After a period of time,the system will stabilize at the boiling temperature for the wash bathand a lower temperature for the rinse bath. Adjustments of boilingtemperature can be made by the addition or removal of HFE7100® from thewash bath in measured amounts, and ultimately the system stabilizes atthe desired wash temperature, i.e., the wash solvent mixture boilingtemperature.

After the temperature has stabilized, and the cards or boards to becleaned have been placed in the basket 28, the basket is lowered throughthe vapor volume or stratum into the boiling wash bath in sump 16 bymeans of hoist assembly 23, to a first depth 63, indicated by dashedlines in FIG. 6, where it is impinged by the pressure jets from wands 41and 42, as shown in FIG. 6, which create a turbulent regiontherebetween. The basket 28 remains in position 63 in the turbulence fora suitable period, e.g., ten minutes, and then is raised by programmedhoist 23, 24 to a second position 64 shown in dashed lines in FIG. 6where it remains for a suitable period, e.g., ten minutes. Basket 28 isthen raised to a third position 66, as seen in FIG. 6, where it remainsfor a period such as, for example, ten minutes, and is then raised abovethe sump into the vapor zone. Thus, the basket 28 is panned past thespray wands 41 and 42 in incremental steps, thereby insuring that theboards contained therein are thoroughly subjected to the sprays from thewands 41 and 42. As discussed hereinbefore, the movements of the basket28 into and out of the wash sump 16, as well as the agitation of thewash solvent within that sump causes some splashing of the liquid. Fence57 substantially prevents the splashed material from reaching the rinseliquid, thereby protecting the rinse liquid in sump 17 fromcontamination.

After completion of the wash cycle, as set forth in the foregoing, thebasket 28 is moved by the hoist assembly 23 to a position over the rinsesump 17, and is then lowered into the rinse liquid. Fence 57 in thisstep functions to prevent excess surge spill-over of the displaced rinseliquid into sump 16, as discussed in the foregoing. After a suitableperiod of time, e.g., five minutes, the basket is raised to the coolzone, defined by cooling coils 53, where it is cooled and then raisedinto the air above the cool zone. The now cooled basket is then loweredinto the vapor zone 63, where the heated vapor condenses on the basketand its contents to produce a vapor rinse. Inasmuch as the rinse agentin the vapor zone is very pure, the vapor rinse is quite effective.

The basket 28 and its contents are then raised into the cool zone, wherethey are again cooled and then into the air above the cool zone beforebeing lowered into the vapor zone again for a second "flushing" by therinse vapor. It has been found in practice that these consecutive vapor"soaks", each of, for example, ninety seconds duration, yield superiorrinsing results. After the last vapor rinse the basket 28 is raised intothe cooling zone for a brief cooling and then can be returned to thestart position, or to whatever position in the system where the basket28 is to be unloaded.

The tank 13 has an unusually high free board so that the volume enclosedthereby is protected from drafts or other air movement in the ambientsurroundings. As a consequence, the vapor zone remains reasonably stillso that there is little escape of rinse agent therefrom.

Table A, appended hereto, details the foregoing process on astep-by-step basis. It is to be understood that various of theparameters discussed, such as times, may be varied somewhat forparticular applications, but the overall process will remainsubstantially the same.

The apparatus and method of the invention, as set forth in the foregoingdescription of a preferred embodiment thereof, achieves excellentcleaning results for circuit boards which, for the most part, exceedestablished or standard criteria, with the use of environmentallyfriendly materials of low toxicity.

In concluding the detailed description, it should be noted that it willbe obvious to those skilled in the art that many variations andmodifications may be made to the preferred embodiment withoutsubstantially departing from the principles of the present invention.All such variations and modifications are intended to be included hereinas within the scope of the present invention, as set forth in theclaims. Further, in the claims, the corresponding structures, materials,acts, process steps, and equivalents of all means or step plus functionelements are intended to include any structure, material, process stepsor acts for performing the functions with other claimed elements asspecifically claimed.

                  TABLE A    ______________________________________         FUNC-    STEP TION    DATA     INPUTS                                DESCRIPTION    ______________________________________    00   RESET    01   IF A>   03      01     Raise Basket    02   GO TO   01      01     Raise Basket    03   S       10:00          Pause 10 seconds    04   CT      0183    02     Move Hoist to Boiling Sump    05   S       05:00          Pause 5 seconds    06   CT      0210    00, 01 Lower Basket into Boiling                                 Vapor    07   S       05:00          Pause 5 seconds    08   CT      0090    00, 01 Lower Basket Into Boiling                                 Liquid    09   M       10:00          Pause 10 minutes    10   CT      0004    01     Raise Basket    11   M       10:00          Pause 10 minutes    12   CT      0006    01     Raise Basket    13   M       10:00          Pause 10 minutes    14   CT      0135    01     Raise Basket    15   S       05:00          Pause 5 seconds    16   CT      0130    02     Move Hoist to Rinse Sump    17   S       10:00          Pause 10 seconds    18   CT      0135    00, 01 Lower Basket into Rinse Liquid    19   M       05:00          Pause 5 minutes    20   CT      0155    01     Raise Basket to Cool Zone    21   S       20:00          Pause 20 seconds    22   CT      0120    01     Raise Basket to Air    23   M       05:00          Pause 5 minutes    24   CT      0205    00, 01 Lower Basket into Rinse Vapor    25   S       90:00          Pause 90 seconds    26   CT      0100    01     Raise Basket to Cool Zone    27   S       20:00          Pause 20 seconds    28   CT      0115    01     Raise Basket to Air    29   M       05:00          Pause 5 minutes    30   CT      0215    01     Lower Basket to Rinse Vapor    31   S       90:00          Pause 90 seconds    32   CT      0100    01     Raise Basket to Cool Zone    33   S       20:00          Pause 20 seconds    34   IF A    >36     01     Raise Basket    35   GO TO   34      01     Raise Basket    36   S       05:00          Pause 5 seconds    37   IF B    >39     00, 02 Return Hoist to Left    38   GO TO   37      00, 02 Return Hoist to Left    39   S       05:00          Pause 5 seconds    40   CT      0060    00, 01 Lower Basket to Start Position    41   S       05:00          Pause 5 seconds    42   GO TO   00             Reset    43    44    45    46    ______________________________________

We claim:
 1. A method for cleaning electronic componentscomprising:filling a first container with a non-ozone depleting washsolvent comprising a mixture of tetradecanoic acid, 1-methyl ethyl esterand dipropylene glycol n-butyl ether to create a wash bath; filling asecond container with a non-ozone depleting rinse solvent comprising amixture of (nonaflourobutyl) methyl ether and (nonaflouroisobutyl)methyl ether to create a rinse bath; maintaining said wash solvent at apredetermined boiling temperature; creating a turbulent region withinsaid wash bath; introducing electronic components to be cleaned into theturbulent region at a first position and holding them there for a firstpredetermined period; raising the components to a second position withinthe turbulent region and holding them there for a second predeterminedperiod; subsequently raising the components to a third position withinthe turbulent region and holding them there for a third predeterminedperiod; removing the components from the wash bath; and rinsing thecomponents in the rinse bath, thereby cleaning said electroniccomponents.
 2. The method as claimed in claim 1 and further includingthe step of establishing the boiling temperature of the wash solvent inthe range of 180° F. to 200° F.
 3. The method as claimed in claim 2wherein the boiling temperature of the wash solvent is within the rangeof 183° F. to 187° F.
 4. The method as claimed in claim 1 wherein thestep of rinsing the components comprises the steps of:introducing thecomponents into the rinse bath for a fourth predetermined period;removing the components from the rinse bath; cooling the components; andintroducing the components into a rinse vapor area to vapor rinse saidelectronic components.